Plant food intake has been consistently associated with lower risk for steroid hormone-dependent cancers. Specific non-nutrient phytochemicals, such as lignans, are similar in structure to sex steroid hormones and may therefore act in vivo to alter hormone metabolism and subsequent cancer risk. The role of diet in the etiology of breast cancer has not yet been determined, but because the majority of postmenopausal breast cancers involve a strong hormonal component, it is probable that phytochemicals may influence this risk. The overall objective of this proposal is to explore the effects of dietary intake of a lignan-rich plant food on circulating endogenous estrogens in postmenopausal women. Specific aims of this proposal are to determine whether: 1) serum concentrations and urinary excretion of sex hormones and binding proteins are influenced by flaxseed ingestion biomarkers of intake within the context of a controlled diet. This study will be implemented as a randomized, crossover flaxseed feeding trial. Thirty postmenopausal women, permanent residents of a convent whose meals are prepared in a centralized kitchen, will be recruited. Three doses, 0, 5, and 10 g/d, of raw flaxseed will be given to subjects each day to be added to their prepared food. Blood will be drawn and urine will be collected over 3 days at the end of each 7-week feeding period, and while subjects are eating their usual diets. Urinary and serum lignans (enterolactone, enterodiol, matairesinol, and secoisolariciresinal) will be measured to determine metabolism and excretion patterns. Serum and urinary sex hormones and sex hormone binding globulin will be measured to determine the effects of varying doses of lignans on hormone profiles. This randomized clinical trial expands our knowledge of women's health and provides more precise information to understand: 1) the epidemiologic associations between plant food intake and breast cancer; 2) the use of diet exposure markers; and 3) potential indicators of breast cancer risk which may be monitored for the primary prevention of cancer.